qa/suites/rados/verify: ping to specific centos

The simple

 os_type: centos

in valgrind.yaml doesn't pick a particular centos, and we end up with
the teuthology default (currently 7.6).

Signed-off-by: Sage Weil <sage@redhat.com>

diff --git a/qa/suites/rados/verify/centos_latest.yaml b/qa/suites/rados/verify/centos_latest.yaml
new file mode 120000 (symlink)
index 0000000..bd9854e
--- /dev/null
+++ b/qa/suites/rados/verify/centos_latest.yaml
@@ -0,0 +1 @@
+.qa/distros/supported/centos_latest.yaml
\ No newline at end of file